Galactic chemical evolution: Hydrogen through zinc

Using the output from a grid of 60 Type II supernova models (Woosley & Weaver 1995) of varying mass (11 ≲ M/M_⊙ ≲ 40) and metallicity (0, 10^-4, 0.01, 0.1, and 1 Z_⊙), the chemical evolution of 76 stable isotopes, from hydrogen to zinc, is calculated. The chemical evolution calculation employs a simple dynamical model for the Galaxy (infall with a 4 Gyr e-folding timescale onto an exponential disk and 1/r² bulge), and standard evolution parameters, such as a Salpeter initial mass function and a quadratic Schmidt star formation rate. The theoretical results are compared in detail with observed stellar abundances in stars with metallicities in the range -3.0 ≲ [Fe/H] ≲ 0.0 dex. While our discussion focuses on the solar neighborhood where there are the most observations, the supernova rates, an intrinsically Galactic quantity, are also discussed. Sampling 4.6 Gyr ago at a distance of 8.5 kpc, we find a composition at the solar circle that is in excellent agreement with the solar abundances from hydrogen to zinc. Type Ia supernovae provide about one-third of solar iron abundance in this distribution. Oxygen comes from the massive stars, but carbon and nitrogen come chiefly from stars with M ≲ 8 M_⊙. The light metal and iron group elements, with the exception of titanium, are in generally good agreement with the stellar abundance data. We also find an age-metallicity relation, a G dwarf distribution, and present-day supernova rates that are in satisfactory agreement with observations. The neutrino process provides a good explanation for the origin of ¹¹B and ¹⁹F and the increase in ⁷Li over its canonical homogeneous big bang value. In order to explain the observed helium-to-metal enrichment (ΔY/ΔZ ≃ 4), we find a favored cutoff in the mass of supernovae that eject all material external to the iron core of about 30 M_⊙, but this limit may be increased by considering the effects of mass loss. The robustness of the results to variations in the iron yields of the Type II supernova models are examined.